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SummaryIn beer fermentation, yeast cells are kept in suspension, despite their higher density, by natural agitation created by ascending CO, bubbles. Yeast cells are unable to nucleate bubbles but instead release Cot in a soluble form in such a way that the medium tends to become supersaturated.A higher concentration of yeast cells and the presence of solid particles cause the formation of bubbles at the bottom of the fermenter and practically only there. The rising bubbles grow and accelerate by sweeping the COZ formed throughout the fermenter by the suspended yeast cells, thereby creating a fluid regime. A mathematical expression relating the bubble agitation power to the fermentation parameters was obtained and used to design more efficient fermenter shapes.The physical action governing the fluid regime of anerobic fermentation has been discovered. The scope of our work is limited to beer fermentation by yeast but we feel that our observations are also valid for other gas producing anaerobic fermentations. Direct observation of the yeast cells and of the C02 bubbles led to the conclusion that the fluid regime is governed by simple and predictable physical phenomena. Yeast having a greater density than its medium sediments immediately after inoculation. But when fermentation starts the evolution of C02 bubbles induces a definite fluid motion. The pattern of this fluid regime is determined by the geometry of the vessel and specifically by the location of the low point. Here in the cloud of sedimented yeast supersaturation and gas evolution occur well before saturation is reached in the main body

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^Vlthough the utilization of ti-paraffins by microorganisms to produce proteins was reported about 75 years ago, only during the last 20 years has interest in this area increased and extensive industrial and academic research effort been under-

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